1. Field of the Invention
This invention relates to an optical recording medium such as an optical recording disc.
2. Prior Art
Optical information media such as read-only optical discs and optical recording discs have been required to have a higher capacity by increasing the recording density for the purpose of recording and storing an enormous amount of information as in the case of motion picture information. Extensive efforts have been dedicated to the research and development of the recording at a higher density to meet such request.
Under such situation, one proposal has been use of a reduced diameter of the laser beam spot in the writing and reading as in the case of DVD (Digital Versatile Disk) by reducing the wavelength used in the writing/reading and increasing the numerical aperture (NA) of the objective lens of the writing/reading optical system. When DVD is compared to CD, DVD has realized a recording capacity (of 4.7 GB/side) which is 6 to 8 times larger than that of the CD by reducing the writing/reading wavelength from 780 nm to 650 nm and increasing the NA from 0.45 to 0.6.
Use of a higher NA, however, invites decrease of tilt margin. Tilt margin is tolerance for the tilting of the optical information medium in relation to the optical system, and the tilt margin is determined by the NA. When the writing/reading wavelength is xcex, and the transparent substrate through which the writing/reading enters the medium has a thickness t, the tilt margin is proportional to
xe2x80x83xcex/(txc2x7NA3)
Tilting of the optical recording medium at an angle to the laser beam, namely, occurrence of the tilt results in the generation of wave front aberration (coma aberration). When the substrate has a refractive index of n and a tilt angle of xcex8, the wave front aberration coefficient is given by
(xc2xd)xc2x7txc2x7{n2xc2x7sinxcex8xc2x7cosxcex8}xc2x7NA3/(n2xe2x88x92sin2xcex8)xe2x88x925/2
These relations indicate that decrease in the thickness t of the substrate is effective when the tilt margin is to be increased and the generation of the coma aberration is simultaneously to be suppressed. As a matter of fact, tilt margin is ensured in the case of DVD by reducing the thickness of the substrate to about half (about 0.6 mm) of the thickness of the CD (about 1.2 mm). In the meanwhile, thickness variation margin of the substrate is given by
xcex/NA4
When the substrate has varying thickness, such variation in the thickness further results in wave front aberration (spherical aberration). When the substrate has a thickness variation of xcex94t, the spherical aberration coefficient is given by
{(n2xe2x88x921)/8n3}xc2x7NA4xc2x7xcex94t
These relations indicate need for reduction of the substrate thickness variation for the purpose of suppressing the spherical aberration associated with the increase in the NA. For example, in the case of DVD, xcex94t is suppressed to xc2x130 xcexcm compared to that of xc2x1100 xcexcm in CD.
A structure enabling further decrease in the substrate thickness has been proposed in order to realize high quality motion picture recording for longer period. In this structure, a substrate having normal thickness is used as a supporting substrate for retaining rigidity of the medium, and the pits and the recording layer are formed on the surface, and a light-transmitting layer in the form of a thin substrate having a thickness of about 0.1 mm is provided on the recording layer. The write/read beam is introduced into the medium through this light-transmitting layer. This structure enables drastic reduction in the thickness of the substrate, and high density recording by the use of a higher NA is thereby enabled. A medium having such structure is described, for example, in Japanese Patent Application Laid-Open No. (JP-A) 320859/1998.
The medium described in JP-A 320859/1998 is a magneto-optical recording medium, and this magneto-optical recording medium has the structure wherein a metal reflective layer, a first dielectric layer, magneto-optical recording layer, a second dielectric layer, and a light-transmitting layer are disposed on the substrate in this order. In JP-A 320859/1998, surface roughness of the metal reflective layer at the interface between the dielectric layer and the metal reflective layer is reduced to the level of less than 8.0 nm based on the view that increase in the noise of the read-out signal is induced by the excessively large surface roughness of the metal reflective layer formed by sputtering. In JP-A 320859/1998, a material containing aluminum, and preferably, a material containing aluminum and in addition, at least one member selected from Fe, Cr, Ti and Si, or gold or silver is used as the material for constituting the metal reflective layer, and ion beam sputtering or magnetron sputtering is employed for the layer formation.
However, from the point of optimizing the recording sensitivity and the reflectivity in practical medium design, it is not preferable that a limitation of the material used for the metal reflective layer is imposed for the purpose of reducing the surface roughness.
It has also been found out in the investigation of the inventors of the present invention that the medium of the constitution described in JP-A 320859/1998 is associated with the risk of defects in the metal reflective layer when the medium is stored under high temperature, high humidity conditions.
An object of the present invention is to reduce noise of the read-out signal and to realize sufficient storage reliability in an optical recording medium comprising a supporting substrate, and a reflective layer, a recording layer, and a light-transmitting substrate disposed on the supporting substrate in this order.
Such objects are attained by the present invention as described in (1) to (4), below.
(1) An optical recording medium to be written and read with a light beam comprising a supporting substrate, and an intermediate layer comprising an amorphous material, a reflective layer comprising a polycrystalline material, a recording layer, and a light-transmitting substrate disposed on the supporting substrate in this order, wherein said medium is used by introducing the write beam and the read beam through the light-transmitting substrate.
(2) The optical recording medium according to the above (1) wherein said intermediate layer has a thickness in the range of 10 to 500 nm.
(3) The optical recording medium according to the above (1) wherein said supporting substrate has guide grooves formed therein, and record marks are formed in the guide groove and/or in the area between adjacent guide grooves.
(4) The optical recording medium according to the above (1) wherein said light-transmitting substrate has a thickness in the range of 30 to 300 xcexcm.